1. Field of the Invention
This invention pertains to compression wave generation. Specifically, the present invention relates to a device and method for indirectly generating a new sonic or subsonic compression wave without the use of a direct radiating element at the source of the new compression wave generation.
2. State of the Art
Sound waves in general are wave-like movements of air or water molecules. Because these media are elastic and generally homogeneous, naturally occurring sound travels in all directions radially from the source of generation. A voice, instrument or impact, for example, will radiate omni-directionally in a unitary, integrated form, carrying multiple frequencies, overtones, and a full range of dynamics that collectively contribute to an instantaneous sound perception at the ear. This perception of naturally occurring sound at a healthy ear is deemed to be "pure" when it corresponds to the same acoustic content that existed at the point of origin.
Because sound is a transient, temporary state of motion within a media, it is not self-sustaining. Indeed, the first and second laws of thermodynamics require that the sound eventually dissipate its motion into heat or other forms of energy. Therefore, if storage or preservation of the sound is desired, it is necessary to transmute such motion into a fixed form of recording. This fixed form can then be recovered later by conversion of the fixed form back into sound waves.
In the earliest experiences of recording, mechanical devices were moved by impact of the sound waves to inscribe or etch a corresponding grove into a plate. By positioning a needle or other tracking device over a set of moving grooves, crude reproduction of the original sound waves was accomplished. More sophisticated technologies have developed which enable capture of sound waves in other fixed forms such as magnetic, electronic, and optical media. Nevertheless, the same principle of sound reproduction has been applied to recover this stored information, whether the response is generated by a mechanical mechanism or by digitally controlled laser reading devices. Specifically, stored signal is converted back to sound waves by recreating movement of an object, which then sets the surrounding air into motion corresponding to sound reproduction.
A primary goal of modern acoustic science is to reproduce pure sound, based on conversion of the electronic, magnetic, mechanical or optical record into compression waves which can be detected at the ear. The ideal system would play all original sound back through a resonating device comparable to that which produced the sound in the beginning. In other words, the violin sounds would be played back through a violin, regenerating the overtones and a myriad of other dynamic influences that represent that instrument. Similarly, a piccolo would be played back through a device that generates the high frequencies, resonance aspects and overtones associated with this type of instrument. In short, one cannot expect a viola to sound like a viola in "pure" form if sound reproduction is actuated by a mechanical wave generating device that does not embody unique characteristics of that instrument or voice. Accordingly, it would seem that the only practical way to reproduce the original "pure" quality of sound would be to isolate each instrument or source, record its sound output, and then reproduce the output into the same instrument or acoustic resonator. It is apparent that such a solution is totally impractical.
In the real world, the challenge of reproducing sound has been allocated to the speaker. The operation of a loudspeaker is relatively simple to understand when the interaction of the components is explained. A speaker is a transducer which receives energy in one form (electrical signals representative of sound) and translates the energy to another form (mechanical vibration). In a dynamic loudspeaker, an electrical current that is proportional to the strength and frequency of the signal to be broadcast is sent through a coil attached to a rigid membrane or cone. The coil moves inside a permanent magnet, and the magnetic field exerts a force on the coil that is proportional to the electrical current. The oscillating movement of the coil and the attached membrane sets up sound waves in the surrounding air. In brief, reproduction of sound has heretofore required mechanical movement of a diaphragm or plate. To expect a single diaphragm or plate to accurately supply both the shrill sound of the piccolo and the deep resonance of the base drum would indeed be unreasonable.
It is important to note, however, that when the listener at a live performance of a symphony hears this broad range of sound, he receives it in an integrated manner as a "unified" combination of sound waves, having a myriad of frequencies and amplitudes. This complex array is responsively promulgated through the air from its originating source to an ear that is incredibly able to transfer the full experience to the brain. Indeed, the full range of audible signal (20 to 20,000 Hz) is processed as a unified experience, and includes effects of subsonic bass vibrations, as well as other frequencies which impact the remaining senses.
It is also important to note that this same "pure" sound that arrives at the ear, can be detected by a microphone and consequently recorded onto a fixed media such as magnetic tape or compact disc. Although the microphone diaphragm may not have the sensitivity of a human ear, modern technology has been quite successful in effectively capturing the full range of sound experience within the recorded signal. For example, it is unnecessary to provide separate microphones for recording both low and high range frequencies. Instead, like the ear drum, the microphone, with its tiny sensing membrane, captures the full audio spectrum as a unified array of sound waves and registers them as a composite signal that can then be recorded onto an appropriate media.
It is therefore clear that the microphone is not the primary limitation to effective storage and subsequent reproduction of "pure" sound. Rather, the challenge of accurate sound reproduction arises with the attempt to transform the microphone output to compression waves through a mechanical speaker. Accordingly, the focus of effort for achieving a high quality unified sound system has been to develop a complex speaker array which is able to respond to high, medium and low range frequencies, combining appropriate resonance chambers and sound coupling devices, to result in a closer simulation of the original sound experience.
This quest for improved sound reproduction has included studies of problems dealing with (a) compensating for the mass of the speaker diaphragm, (b) the resistance of air within an enclosed speaker, (c) the resonant chamber configuration of the speaker, (d) the directional differences between high and low frequencies, (e) the phase variation of low versus high frequency wave trains, (f) the difficulty of coupling speaker elements to surrounding air, and (g) the loss of harmonics and secondary tones. Again, these aspects represent just a few of the problems associated with reconstructing the sound wave by means of a direct radiating physical speaker.
As an example of just one of these issues, overcoming the mass of a speaker driver has remained a challenging problem. Obviously, the purpose of the speaker driver and diaphragm is to produce a series of compression waves by reciprocating back and forth to form a wave train. The initial design challenge is to compensate for resistance against movement in speaker response due to inertia within the speaker mass itself. Once the speaker driver is set in motion, however, the mass will seek to stay in motion, causing the driver to overshoot, requiring further compensation for delayed response to reverse its direction of travel. This conflict of mass and inertia recurs thousands of times each second as the speaker endeavors to generate the complex array of waves of the original sound embodied in the electrical signal received.
In order to meet the difficulty of compensating for mass, as well as numerous other physical problems, speaker development has focused mainly on improving materials and components as opposed to developing a different concept of sound generation. Diaphragm improvements, cone construction materials, techniques and design, suspensions, motor units, magnets, enclosures and other factors have been modified and improved. Nevertheless, the basic use of a reciprocating mass remains unchanged, despite an efficiency of less than 5 percent of the electrical power being converted to acoustic output.
Electrostatic loudspeakers represent a different methodology. Unlike the electrodynamic loudspeaker with its cone shaped diaphragm, the electrostatic loudspeaker uses a thin electrically conducting membrane. Surrounding the plate are one or more fixed grids. When a signal voltage is applied to the elements, the electrostatic force produced causes the diaphragm to vibrate. This low-mass diaphragm is particularly useful as a high-frequency radiating element, and its operation can be extended to relatively low frequencies by the use of a sufficiently large radiating area.
Although electrostatic speakers offer some advantages, they are large, expensive, inefficient and suffer from the lack of point source radiated sound. For example, sound detection is accomplished by a microphone at a localized or approximate point source. To convert the detected sound to a non-point source, such as a large electrostatic diaphragm, may create unnatural sound reproduction. Specifically, a radiating electrostatic speaker 5 feet in height is limited in its ability to simulate the delicate spatial image of a much smaller piccolo or violin.
Another issue in loudspeaker design is that the optimum mass and dimensions for low frequency radiating elements differ radically from those for high frequency. This problem is typically addressed by providing both woofer and tweeter radiating elements for each channel of a loudspeaker system. The implications of this design are highly undesirable. The phase shift introduced because of the differences in time delay for high frequency signals traveling (i) the shorter distance of the cone of a tweeter to a listener, versus (ii) the substantially longer path for low frequency signals from the horn or woofer speaker to a listener's ear, can be in the range of thousands of percent in phase differential.
The preceding discussion of speaker technology is recited primarily to emphasize the historical difficulty of changing a stored form of sound to a compression wave capable of reproducing sound in its original form. Nevertheless, the prior art has been virtually dominated for sixty years by the concept that mechanical systems, such as speakers, are required to reproduce audible sound. Clearly, it would be very desirable to provide a means of sound reproduction which adopts a different approach, avoiding the many difficulties represented by the choice of moving a diaphragm or speaker in order to generate sound.